Thoracic Aorta - How to Measure

Dilatation of the thoracic aorta is often incidentally detected during transthoracic echocardiography (TTE) or chest CT performed for unrelated indications.

For initial diagnosis and confirmation of aortic root or proximal ascending aorta dilatation measurements should be obtained using cardiac CT (CCT).

In certain cases, cardiac magnetic resonance imaging (CMR) is the preferred modality, particularly in young patients or those requiring long-term follow-up.
Routine follow-up is typically performed using CCT; however, TTE may be utilized if there is no significant discrepancy with CCT measurements and the degree of dilatation is moderate.

TTE measurements

TTE measurements of the thoracic aorta are obtained using the leading edge-to-leading edge technique in end-diastole on a parasternal long-axis view.

This refers to the measurement of the diameter from the interface between the right ventricular outflow tract and the anterior aortic wall to the interface of the aortic lumen with the posterior aortic wall, as indicated by the arrows.
On TTE it is not possible to reliably measure the thickness of the posterior aortic wall.

Matter of Millimeters

Accurate measurement of the thoracic aorta's diameter is a matter of millimeters.

To demonstrate the importance of precise measurements, the table is presented showing one of the many follow-up protocols that exsist for dilatation of the thoracic aorta.
Do not study this table, it is only to show you that a difference of just 2 mm can reclassify a patient into a distinct risk category, thereby altering their follow-up protocol or surgical indication.

Normal diameters

When evaluating the thoracic aorta, measurements are typically obtained at seven standard anatomical levels, though some protocols may include additional points for enhanced precision.

The aortic root is assessed at three distinct levels:

• Aortic annulus
• Sinuses of Valsalva
• Sinotubular junction

Further measurements may be taken at the following levels:

• Proximal aortic arch
• Mid aortic arch
• Descending aorta (with some protocols including two separate measurements)
• Abdominal aorta

Normal reference values for these measurements are provided in the table for men (in blue) and women (in pink).

Double-oblique method

The thoracic aorta follows a curved, oblique path.
Axial slices which are perpendicular to the long axis of the body do not align with the true cross-section of the aorta, leading to elliptical or distorted views rather than circular ones (red ellipse).

The double-oblique method provides the true maximal diameter of the thoracic aorta by aligning the imaging plane perpendicular to the long axis of the aorta.

Technique

• First Oblique Plane: The aorta is visualized in a sagittal or coronal oblique plane to identify its long axis.
• Second Oblique Plane: A perpendicular plane is then created at the level of interest (e.g., ascending aorta, aortic arch, or descending aorta). This plane is orthogonal to the long axis of the aorta, ensuring a true cross-sectional view.

Aortic root

1. Aortic annulus: fibrous ring at the base of the aortic valve, where the valve leaflets (cusps) attach.
2. Sinus of Valsalva: three pouches formed by the aortic wall at the level of the aortic valve cusps.
   They create swirling blood flow, that aid in the efficient closure of the aortic valves.
   The right coronary artery originates from the right sinus of Valsalva, and the left coronary artery originates from the left sinus.
   
3. Sinotubular junction: narrowest part of the aortic root, marking the boundary between sinus of Valsalva and ascending aorta.

Aortic annulus

In CT imaging, the annulus is typically measured on the slice just below the lowest visible insertion points (nadirs) of the aortic valve cusps (see video).

This is often referred to as the "annular plane" or "hinge plane".
The last slice where the leaflets are still visible is usually above the annular plane.
Make sure that you see all three insertion points in one plane.
The measurement is taken one slice below this level to capture the true annular dimensions.

Measuring at this level ensures consistency for prosthetic valve sizing (e.g., TAVR/TAVI).

Sinus of Valsalva

The diameter is measured at the level of maximal sinus bulging, identified by scrolling upward from the annulus to the widest point.

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Measurement Methods

1. Sin-Com (Sinus-to-Commissure):
   A line is drawn from the deepest point of one sinus perpendicularly through the center to the opposite commissure. This yields a precise but smaller dimension, often underestimating the true aortic root diameter.
2. Mid Sin-Sin (Mid-Sinus to Mid-Sinus):
   A line connects the midpoints of opposite sinuses of Valsalva.
3. Max Sin-Sin (Maximum Sinus-to-Sinus):
   A line is drawn from the center of the deepest sinus to the center of an adjacent sinus. The average of the three sinus-to-sinus dimensions should be calculated, or the largest dimension can be reported.

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Note: These methods produce different results. 
We prefer the sinus-to-sinus method. I
Every institutions should standardize and clearly document their measurement approach.

Video of Root Aneurysms

Elongation of Ascending Aorta

Scroll through the images to see how to measure the thoracic aorta using MPR reconstructions.